The invention relates to a process for the controlled modification of the geometrical characteristics of the endmost face of a monomode optical fiber particularly with a view to providing optimized coupling with another monomode optical element, this element being able to be without distinction another optical fiber, an integrated wave-guide or a laser.
Such devices are more especially used in optical connection systems. These systems are used for providing optical path telecommunications in numerous fields. By way of non limiting examples there may be mentioned telephone or telex transmission, telematics or teledistribution.
For transmissions over long distances, very low loss optical fibers are used. Particular care must be taken with couplings between optical fibers on the one hand and, on the other hand, between the optical fibers and the opto-electronic means for emitting and detecting the radiant energy transmitted by these optical fibers.
As is known, the connection may be of the multimode or monomode type, these latter allowing connection with a higher flowrate and lower attenuation over long distances.
For these applications, efforts have then been made to form optical systems comprising only monomode elements such as semiconductor lasers, integrated wave-guides or monomode optical fibers.
One of the fundamental problems which arises in a monomode transmission chain is that of coupling between the different monomode elements, this is all the more acute than when multimode elements are used. In fact, in each element a mode is excited which is not necessarily adapted to the next element in the chain. Thus, so as to couple a maximum of energy into the downstream monomode element, the upstream mode must be modified and matched, or the reverse.
A particularly delicate example of coupling to be a achieved is the coupling between a divergent radiation source, particularly a transverse monomode semi-conductor laser, and a monomode optical fiber. Direct coupling, in which the source and the fiber are simply placed face to face without an intermediate optical element is of low efficiency because of the reduced dimensions of the fiber and of the generally large aperture of the laser beam. So as to increase this efficiency, different optical configurations have been studied on the basis of cones, cylindrical, semicylindrical or spherical lenses, hemispherical lenses. Theoretical and practical studies show that interesting results can be obtained with a hemispherical lens coupled to the end of the fiber. Different manufacturing methods have been proposed. Among these, thermo-forming, which consists in melting the end of the fiber by any means, has been described more particularly in two articles published in 1975 in the review "Applied Optics": vol. 14, no. 2, pages 294-298 and vol. 14, no. 12, pages 2815-2816. This technique often requires, especially for the silica fibers commonly used, very high temperatures, so heavy apparatus.
A simpler solution to this problem was described in French patent application published under the no. 2 465 238 entitled: "Dispositif de couplage entre une source lumineuse de rayonnement divergent et une fibre optique at procede de realisation d'un tel dispositif".
This patent application relates to the construction of a plano-convex lens made from high index glass on any type of fiber. The coupling device comprises a plano-convex lens coupled to the input face of the fiber, formed from a glass having a refraction index greater than that of the fiber and obtained by placing the end of the fiber in contact with a drop of molten glass. A spherical skul-cap shape is obtained whose parameters are controllable.
Although simpler to use than the previously known processes, this process requires nevertheless the creation of an added element, foreign to the fiber: the plano-convex lens; and having a geometry defined once and for all: demi-sphere.
On the contrary, the invention proposes a process for elaborating a phase object on the endmost face of the fiber which remains simple to use while permitting increased flexibility with respect to the processes of the prior art. Moreover, this process does not require the addition of external elements to the fiber, the modification made to the fiber for the elaboration of said phase object being carried out in a continually controllable way.
The invention provides then a process for the controlled modification of the geometrical characteristics of the end of a monomode optical fiber, the fiber being formed from a central region and at least one peripheral region having different optical refraction indices, these index differences being obtained by incorporating in a basic material in a given percentage at least one doping material, which process comprises the following steps:
previous preparation of the end of said fiber so as to obtain an endmost face free from splinters;
chemical etching for a pre-determined period of time at differentiated rates of said regions with given doping percentages at the end of the fiber;
neutralization of the etching;
injection into the fiber of guided radiation and control of the state of modification of said geometrical characteristics of the end of the fiber by plotting the variations of the profile of the remote field of said radiation radiated by this etched end.
The invention has further as object the application of this modification process to the coupling of a monomode optical fiber with another monomode element.